A battery stores electrochemical energy by separating an ion source and an ion sink at differing ion electrochemical potential. A difference in electrochemical potential produces a voltage difference between the positive and negative electrodes; this voltage difference will produce an electric current if the electrodes are connected by a conductive element. In a battery, the negative electrode and positive electrode are connected by two conductive elements in parallel. The external element conducts electrons only, and the internal element (electrolyte) conducts ions only. Because a charge imbalance cannot be sustained between the negative electrode and positive electrode, these two flow streams supply ions and electrons at the same rate. In operation, the electronic current can be used to drive an external device. A rechargeable battery can be recharged by application of an opposing voltage difference that drives electronic current and ionic current in an opposite direction as that of a discharging battery in service. Thus, the active materials of rechargeable batteries need to be able to accept and provide ions. Increased electrochemical potentials produce larger voltage differences the cathode and anode, and increased voltage differences increase the electrochemically stored energy per unit mass of the device. For high-power devices, the ionic sources and sinks are connected to the separator by an element with large ionic conductivity, and to the current collectors with high electronic conductivity elements.
Rechargeable batteries can be constructed using static negative electrode/electrolyte and positive electrode/electrolyte media. In this case, non-energy storing elements of the device comprise a fixed volume or mass fraction of the device; thereby decreasing the device's energy and power density. The rate at which current can be extracted is also limited by the distance over which cations can be conducted. Thus, power requirements of static cells constrain the total capacity by limiting device length scales.
Redox flow batteries, also known as a flow cells or redox batteries or reversible fuel cells are energy storage devices in which the positive and negative electrode reactants are soluble metal ions in liquid solution that are oxidized or reduced during the operation of the cell. Using two reversible redox couples, liquid state redox reactions are carried out at the positive and negative electrodes. A redox flow cell typically has a power-generating assembly comprising at least an ionically transporting membrane separating the positive and negative electrode reactants (also called catholyte and anolyte respectively), and positive and negative current collectors (also called electrodes) which facilitate the transfer of electrons to the external circuit but do not participate in the redox reaction (i.e., the current collector materials themselves do not undergo Faradaic activity). Redox flow batteries have been discussed by C. Ponce de Leon, A. Frias-Ferrer, J. Gonzalez-Garcia, D. A. Szantos and F. C. Walsh, “Redox Flow Batteries for Energy Conversion,” J. Power Sources, 160, 716 (2006), M. Bartolozzi, “Development of Redox Flow Batteries: A Historical Bibliography,” J. Power Sources, 27, 219 (1989), and by M. Skyllas-Kazacos and F. Grossmith, “Efficient Vanadium Redox Flow Cell,” Journal of the Electrochemical Society, 134, 2950 (1987).
Differences in terminology for the components of a flow battery and those of conventional primary or secondary batteries are herein noted. The electrode-active solutions in a flow battery are typically referred to as electrolytes, and specifically as the catholyte and anolyte, in contrast to the practice in lithium ion batteries where the electrolyte is solely the ion transport medium and does not undergo Faradaic activity. In a flow battery, the non-electrochcinically active components at which the redox reactions take place and electrons are transported to or from the external circuit are known as electrodes, whereas in a conventional primary or secondary battery they are known as current collectors.
While redox flow batteries have many attractive features, including the fact that they can be built to almost any value of total charge capacity by increasing the size of the catholyte and anolyte reservoirs, one of their limitations is that their energy density, being in large part determined by the solubility of the metal ion redox couples in liquid solvents, is relatively low. Methods of increasing the energy density by increasing the solubility of the ions are known, and typically involve increasing the acidity of the electrode solutions. However, such measures which may be detrimental to other aspects of the cell operation, such as by increasing corrosion of cell components, storage vessels, and associated plumbing. Furthermore, the extent to which metal ion solubilities may be increased is limited.
In the field of aqueous electrolyte batteries, and specifically batteries that utilize zinc as an electroactive material, electrolytes that comprise a suspension of metal particles and in which the suspension is flowed past the membrane and current collector, have been described. See for example U.S. Pat. Nos. 4,126,733 and 5,368,952 and European Patent EP 0330290B1. The stated purpose of such electrodes is to prevent detrimental Zn metal dendrite formation, to prevent detrimental passivation of the electrodes, or to increase the amount of zincate that can be dissolved in the positive electrode as the cell discharges. However, the energy density of such fluidized bed batteries even when electrolytes with a suspension of particles are used remains relatively low.
Thus, there remains a need for high energy-density and high power-density energy storage devices.